The H+ -K+ -ATPase alpha2 (HKalpha2) gene expressed in kidney and colon plays a critical role in the maintenance of body potassium and acid-base balance during chronic hypokalemia and chronic sodium depletion. The broad objectives of the proposed research are to identify the molecular mechanisms underlying the transcriptional regulation of this gene in the kidney and colon during these commonly encountered clinical conditions. Data are presented to indicate that the HKalpha2 gene is differentially regulated at the transcriptional level in the renal outer medulla and distal colon in response to chronic hypokalemia and states of aldosterone excess, respectively. The proposal examines the hypothesis that specific transcription factors selectively expressed or induced in these tissues interact with cis- regulatory elements to control HKalpha2 gene expression. The structural organization and precise chromosomal position of the murine HKalpha2 gene will be characterized. The selective effects of mineralocorticoids and glucocorticoids on HKalpha2 transcription in the distal colon and kidney will be analyzed to determine whether transcriptional induction is specific to the cognate nuclear receptors for these ligands. DNase I hypersensitivity assays and in vivo footprinting coupled with analysis of HKalpha2 regulatory region-reporter gene constructs will be used to map, at single nucleotide resolution, cell- specific promoter and enhancer elements in epithelial cell lines derived from the renal medullary collecting duct and distal colon under normal conditions and in response to low external K+ and aldosterone. Gel shift and supershift assays of nuclear extracts will be used to identify further the trans-acting factors responsible for HKalpha2 transcriptional control. The ability of the nuclear proteins to alter HKalpha2 promoter/enhancer in trans will be tested in coexpression experiments. Studies in transgenic mice will test whether candidate regulatory elements identified in vitro faithfully mirror the tissue expression and responses to chronic K+ - or Na+-deprivation, and aldosterone excess of the endogenous HKalpha2 gene. The results of these studies should provide important molecular insights into the regulation of the HKalpha2 gene and its unique roles in renal and intestinal cell biology and pathobiology. The proposed studies should also yield fundamental information that can be more broadly applied to the Na+-K+-ATPase/H+-K+-ATPase multi-gene family, the molecular basis for tissue-specific gene regulation, and the molecular mechanisms responsible for selective steroid action on target genes.